Not Applicable.
The present invention relates to a process for the catalytic partial oxidation of hydrocarbons (e.g., natural gas), and in particular to a process for oxidizing methane to produce a mixture of carbon monoxide and hydrogen using a bulk rhodium catalyst in the form of metal cloth, such as gauze or felt.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming is currently the major process used commercially for the conversion of methane to synthesis gas, and proceeds according to Equation 1.
CH4+H2O⇄CO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to its significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
CH4+xc2xdO2xe2x86x92CO+2H2xe2x80x83xe2x80x83(2)
This ratio is more useful for the downstream conversion of the syngas to chemicals such as methanol and to fuels than the H2:CO ratio steam reforming. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes.
The syngas produced by either steam reforming or partial oxidation may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors. One of the most important factors is the choice of catalyst structure. For successful operation on a commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, while maintaining high selectivity of the process to the desired products of carbon monoxide and hydrogen. Accordingly, there has been an effort to investigate catalysts that provide high selectivity for specified products and have structures that promote partial oxidation of hydrocarbons at high gas hourly space velocities, such as metal gauze catalysts.
Certain specialized catalysts are known to be suitable for other, unrelated catalytic reactions. For example, platinum/rhodium alloy gauze catalysts, typically about 5-15 wt % rhodium, are used industrially for the catalytic synthesis of hydrogen cyanide, as well as for the catalytic oxidation of ammonia to nitric acid. In addition to allowing high gas throughput, gauze catalysts are typically mechanically durable. Platinum/rhodium catalysts used for hydrogen cyanide and nitric acid synthesis typically have lives of several months before being removed for remanufacturing by reclaiming up to 99% of the metal and using it to make new catalyst. A characteristic of platinum/rhodium catalysts however is a tendency to undergo surface rearrangement. It is known in the art that surface rearrangement contributes to the reduction of catalyst life due to pore plugging. A typical form of surface rearrangement of platinum/rhodium alloy catalyst, for example when used for ammonia oxidation, is the formation of dendritic excrescences, as disclosed in xe2x80x9cStuctured Catalysts and Reactors,xe2x80x9d edited by A. Cybulski and J. A. Moulijn, 1998, pp. 61-66, hereby incorporated herein by reference.
Similar platinum/rhodium alloy gauze catalysts have been disclosed as having utility for the synthesis of a mixture of synthesis gas and formaldehyde, such as is disclosed in U.S. Pat. No. 5,654,491 and European Patent 064559. A process that includes formaldehyde formation has the disadvantage of less selectivity to synthesis gas formation.
Some platinum group gauze catalysts have been studied as hydrogen synthesis catalysts. For example, M. Fathi et al., Catal. Today, 42, 205-209 (1998) disclose the catalytic partial oxidation of methane over Pt, Pt/Rh, Pt/Ir and Pd gauze catalysts at contact times of 0.00021 to 0.00033 seconds. Single gauze catalysts were tested in a quartz reactor at 700xc2x0 C. to 1100xc2x0 C. and it was observed that, although high selectivities to carbon monoxide were observed at high temperatures, the selectivity to hydrogen gas was low, xe2x80x9cbelow 30% in most cases.xe2x80x9d
Further, K. Heitnes Hofstad, et. al. Catalysis Letters 36, 25-30 (1996) disclose: xe2x80x9cPartial oxidation of methane has been studied on a Pt gauze catalyst under conditions where the conversion of O2 was not complete. The results show that at these very low space times high selectivities of CO are obtained, but low selectivities of H2 (are obtained) even at temperatures above 800xc2x0 C.xe2x80x9d
Notwithstanding the foregoing patents and teachings, there remains a need for a process for the partial oxidation of hydrocarbons using a long-lived, durable catalyst suited to produce synthesis gas with high conversions of methane, high selectivities to both CO and H2, and with high gas throughput.
The present invention provides a catalyst and a process for the catalytic partial oxidation of a hydrocarbon feedstock by contacting a feed stream comprising the hydrocarbon feedstock and an oxygen-containing gas with a catalyst in a reaction zone maintained at conversion-promoting conditions that are effective to produce an effluent stream comprising carbon monoxide and hydrogen. The catalyst preferably includes high surface area bulk rhodium. More preferably, the catalyst is in the form of at least one layer of rhodium cloth. As used herein and described in detail below, the term xe2x80x9crhodium clothxe2x80x9d refers to a mechanically fixed arrangement of metal wire in a substantially planar configuration and is intended to include rhodium gauze and rhodium felt.
The present catalysts are preferably pretreated to activate the catalyst by oxidation in air at a temperature of between about 300xc2x0 C. and about 1200xc2x0 C., preferably between about 900xc2x0 C. and about 1000xc2x0 C. Preferably, the air oxidation pretreatment is carried out for a period of time of about one half to four hours.
The preferred catalysts allow effective partial oxidation of the feed gas with high selectivity, together with high methane conversion. CO and H2 selectivities of at least 90% and methane conversion rates of at least 90% are achieved. Further, the preferred catalysts are durable and have long lived activity. The preferred catalysts can typically be used without replacement for a period of at least about six months.
The present gauze and felt catalysts are preferably made of wire of about 0.025 to about 0.25 mm diameter, preferably about 0.1 mm diameter. The gauze catalyst preferably includes two or more layers of gauze, and more preferably from about 5 to about 100 layers. A preferred gauze layer is about 20 to about 100 mesh, preferably about 40 to about 80 mesh. Correspondingly, felt densities are from about 10% to about 50%, preferably 35%. The felt thickness is preferably from about 5 mm to about 50 mm.
The preferred ranges of operating conditions for using the present catalysts include temperatures of about 800xc2x0 C. to about 1300xc2x0 C., preferably from about 1000xc2x0 C. to about 1200xc2x0 C., and pressures of about 110 kPa to about 3000 kPa, preferably from about 500 kPa to about 3000 kPa. In addition, the feed stream is preferably preheated to from about 50xc2x0 C. to about 700xc2x0 C., and preferably to about 400xc2x0 C., before contacting the catalyst. Typical space velocities for the process are from about 60,000 hrxe2x88x921 to about 20,000,000 hrxe2x88x921, and are preferably from about 150,000 hrxe2x88x921 to about 10,000,000 hrxe2x88x921.